The present invention is generally directed to a heterodyne optical spectrum analyzer. More particularly, the present invention is directed to a heterodyne optical spectrum analyzer having provisions for compensation of asymmetries of a heterodyne receiver based upon measurements of a known balancing tone (b-tone) input signal.
Because of their high resolution, heterodyne optical spectrum analyzers are useful for spectral analysis of dense wavelength division multiplexing (DWDM) systems. Unfortunately, the presence of intensity noise within each optical channel of a DWDM system can degrade signal quality output by heterodyne optical receivers. As the number of channels incident on a heterodyne receiver/detector increases, so does the level of intensity noise that the heterodyne receiver must suppress.
With reference to FIG. 1 a known optical receiver 1 for reducing intensity noise is disclosed. In this known system an optical coupler 6 receives an input signal and an local oscillator signal (LO) and mixes, or combines, the two signals to output mixed/combined optical signals which are detected by two photodiodes 2 and 3. Photodiodes 2 and 3 are connected in series. The connection point 4 serves as a virtual ground, which allows for common mode rejection of intensity noise. This solution provides typically 20 to 30 dB of noise reduction which may be adequate where a relatively small number of available optical channels of a DWDM system are presented to the optical receiver 1, for example ten (10) channels or fewer. As the presence of DWDM channels increases a means of achieving better noise subtraction is needed.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.
The present invention provides a heterodyne optical spectrum analyzer having provisions for subtracting noise presented to it by a dense wavelength division multiplexing (DWDM) signal or system.
Briefly described, in architecture, the system can be implemented as follows. A local oscillator is provided for generating a local oscillator signal. There is provided a balancing tone generator for generating an optical balancing tone (b-tone). A balanced heterodyne optical receiver having a first and a second channel is provided for receiving the b-tone signal and outputting two electrical signals in response to the optical b-tone. A measuring unit is provided for measuring the electrical output signals and determining their characteristic transfer functions.
The present invention can also be viewed as providing a method for analyzing an optical signal in which a balancing tone is injected into an optical path and input to a balanced heterodyne optical receiver. Electrical signals output from the balanced heterodyne optical receiver in response to the input balancing tone are measured to determine their characteristics. Based upon differences in the measured characteristics of the electrical output signals, a compensating transfer function is determined and applied to the signal path to offset the measured differences.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention.